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Why don’t stones burn? – Luke, age 4, New Market, New Hampshire
Although many stones do not burn, some do. It depends on what the rocks are made of – and that depends on how they are formed.
There are three main types of rocks: igneous, sedimentary, and metamorphic. These rocks are made of minerals that all have different characteristics. Some will melt into magma or lava – super-hot, liquid rock – when exposed to heat. Others will catch fire.
Rocks that burn when heated are igniting. This means that elements in the rocks react with oxygen in the air to produce heat and light in the form of flames.
The elements sulfur, carbon and hydrogen react easily with oxygen. Rocks containing these elements are flammable. Without these elements, rocks exposed to enough heat will melt instead of catching fire.
How rocks are formed
Igneous rocks are formed when magma underground or lava from a volcano cools and crystallizes into solid material. These rocks are usually made of silicate minerals that crystallize at temperatures ranging from 1300 degrees Fahrenheit (700 degrees Celsius) to as high as 2400 F (1300 C).
Igneous rocks contain few or no combustible elements. And it is very difficult to melt them back into magma, because they crystallize at such high temperatures. Making that possible requires the kind of high-tech incinerator that cities use to burn waste.
Sedimentary rocks have a very different formation story. They form from broken pieces of rock, minerals, sometimes plant or animal matter, and also crystals left behind when water evaporates, such as the limescale that forms in tea kettles and bathtubs.
There is a lot of sulphur, carbon and hydrogen in living things. In fact, these are three of the six essential elements of life on Earth. Bits of organic matter, especially dead plants, are also flammable and cause the rocks to burn.
The last group of rocks is called metamorphic because these rocks form when a lot of heat and pressure change existing rocks into new types without melting or burning them. ‘Metamorphosis’ comes from Ancient Greek and means ‘transformation’. The marble you see in kitchen countertops or statues, for example, comes from limestone that has been transformed deep underground under intense heat and pressure.
The rock that people burn: coal
Metamorphic rocks formed from igneous rocks do not contain the combustible elements (the ones that burn), but metamorphic rocks made from sedimentary rocks may. A well-known example is anthracite coal, which consists almost entirely of carbon. It was created when dead plants fell into swamps long, long ago, were buried by sand or mud, and eventually compressed into coal over hundreds of millions of years.
There are many coal seams around the world. Sometimes the coal even catches fire while it is still in the ground. The cause can be natural, such as a lightning strike or human activities such as mining.
In Centralia, Pennsylvania, a former mining town, a coal seam has been burning for more than 50 years. There are other active coal seam fires in places around the world, including Zimbabwe in Africa and Jharia in India.
If carbon is compressed with even more pressure than needed to make coal, you end up with diamonds – the hardest mineral found in nature. In 1772, French chemist Antoine Lavoisier proved that diamonds could ignite when he burned one with a magnifying glass.
With enough patience you could burn a diamond in a candle flame. But since diamonds are quite expensive, it is better to burn other things made of carbon, such as leaves under a magnifying glass, or sticks and marshmallows in a campfire.
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This article is republished from The Conversation, an independent nonprofit organization providing facts and trusted analysis to help you understand our complex world. It was written by: Natalie Bursztyn, University of Montana
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Natalie Bursztyn does not work for, consult with, own shares in, or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.